Urea (NH2CONH2) can be produced from ammonia and carbon dioxide at elevated temperature (typically between 150° C. and 250° C.) and pressure (typically between 12 and 40 MPa) in the synthesis zone of a urea plant. In this synthesis, two consecutive reaction steps can be considered to take place. In the first step ammonium carbamate is formed, and in the next step, this ammonium carbamate is dehydrated so as to give urea, The first step (i) is exothermic, and the second step can be represented as an endothermic equilibrium reaction (ii):2NH3+CO2→H2N—CO—ONH4  (i)H2N—CO—ONH4⇄H2N—CO—NH2+H2O  (ii)
In a typical urea production plant, the foregoing reactions are conducted in a urea synthesis section so as to result in an aqueous solution comprising urea. In one or more subsequent concentration sections, this solution is concentrated to eventually yield urea in a form of a melt rather than a solution. This melt is further subjected to one or more finishing steps, such as prilling, granulation, pelletizing or compacting.
A frequently used process for the preparation of urea according to a stripping process is the carbon dioxide stripping process as for example described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. In this process, the synthesis section followed by one or more recovery sections. The synthesis section comprises, a reactor, a stripper, a condenser and but not necessarily, a scrubber in which the operating pressure is in between 12 and 18 MPa and preferably in between 13 and 16 MPa. In the synthesis section the urea solution leaving the urea reactor is fed to a stripper in which a large amount of non-converted ammonia and carbon dioxide is separated from the aqueous urea solution. Such a stripper can be a shell and tube heat exchanger in which the urea solution is fed to the top part at the tube side and a carbon dioxide feed to the synthesis is added to the bottom part of the stripper. At the shell side, steam is added to heat the solution. The urea solution leaves the heat exchanger at the bottom part, while the vapor phase leaves the stripper at the top part. The vapor leaving said stripper contains ammonia, carbon dioxide, inert gases and a small amount of water. Said vapor is condensed in a falling film type heat exchanger or a submerged type of condenser that can be a horizontal type or a vertical type. A horizontal type submerged heat exchanger is described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The heat released by the exothermic carbamate condensation reaction in said condenser is usually used to produce steam that is used in a downstream urea processing section for heating and concentrating the urea solution. Since a certain liquid residence time is created in a submerged type condenser, a part of the urea reaction takes already place in said condenser. The formed solution, containing condensed ammonia, carbon dioxide, water and urea together with the non-condensed ammonia, carbon dioxide and inert vapor is sent to the reactor. In the reactor the above mentioned reaction from carbamate to urea approaches the equilibrium. The ammonia to carbon dioxide molar ratio in the urea solution leaving the reactor is generally in between 2.5 and 4 mol/mol. It is also possible that the condenser and the reactor are combined in one piece of equipment. An example of this piece of equipment as described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The formed urea solution leaving the urea reactor is supplied to the stripper and the inert vapor comprising non-condensed ammonia and carbon dioxide is sent to a scrubbing section operating at a similar pressure as the reactor. In that scrubbing section the ammonia and carbon dioxide is scrubbed from the inert vapor. The formed carbamate solution from the down stream recovery system is used as absorbent in that scrubbing section. The urea solution leaving the stripper in this synthesis section requires a urea concentration of at least 45% by weight and preferably at least 50% by weight to be treated in one single recovery system downstream the stripper. The recovery section comprises a heater, a liquid/gas separator and a condenser. The pressure in this recovery section is between 0.2 to 0.5 MPa. In the heater of the recovery section the bulk of ammonia and carbon dioxide is separated from the urea and water phase by heating the urea solution. Usually steam is used as heating agent. The urea and water phase, contains a small amount of dissolved ammonia and carbon dioxide that leaves the recovery section and is sent to a downstream urea processing section where the urea solution is concentrated by evaporating the water from said solution. The vapor released in the heater of the recovery system comprises ammonia, carbon dioxide and water. Said vapor is condensed in a condenser. The heat of condensation is dissipated in cooling water. The formed carbamate is used as absorbent in said scrubber in the synthesis section. Some non-condensed vapor comprising ammonia, carbon dioxide and inert leaving that scrubber is sent to a condenser or absorber in order to purify the inert before releasing it into the atmosphere. The pressure in said condenser and/or absorber is typically lower than the pressure in the synthesis section.
An inherent consequence of the production of urea, is the unwanted emission of ammonia, particularly as a result of unreacted ammonia leaving the synthesis zone. Also in the most modern urea plants, this emission cannot be avoided, save for a prohibitive energy input and ditto operating costs to separate and capture all of the ammonia.
E.g., in a typical urea melt plant according the CO2 stripping process, continuous ammonia emissions take place on the following process emission points:
low pressure absorber;
atmospheric absorber;
breathing system of the urea solution storage;
breathing system of the process condensate storage
Thus, at several instance of the production of a urea melt, ammonia emissions occur. Whilst some may be discontinuous, a focus is on further reducing, and preferably avoiding, the continuous ammonia emissions.
The state of the art technology to minimize ammonia emissions from urea melt plants, is based on the “end-of-pipe” technology of “flaring”. Especially for continuous ammonia emission reduction, flaring is a costly solution since flaring of these continuous emission sources requires relative large amounts of support gas and nitrogen to prevent explosive vapor mixtures caused by oxygen ingress via the flare tips. Besides, flaring gives a secondary emission by, e.g., nitrogen oxygen (NOx formation.